1. Field of the Invention
The invention relates to method of manufacturing a reticle used in a photolithography step which is one of the steps for manufacturing a semiconductor integrated circuit device.
2. Description of the Related Art
In manufacturing a semiconductor integrated circuit device, a photolithography step is adopted for forming a micro pattern with high accuracy on a semiconductor wafer. In the photolithography step, there is used a so-called "reticle". The reticle is composed of a glass substrate such as quartz on which is formed a circuit pattern composed of material such as chromium which prevents ultraviolet rays from transmitting therethrough. The reticle is set in a stepper which is an apparatus for exposing a semiconductor wafer to light, and the circuit pattern formed on the reticle is imaged by the stepper to a semiconductor substrate to thereby fabricate an integrated circuit device.
FIG. 1 schematically illustrates a stepper. A semiconductor wafer W, which will form a semiconductor integrated circuit device, is placed on an X-Y stage 31 which is movable both in X and Y directions. On an upper surface of the wafer W is formed an electrically conductive layer comprising a circuit, and on the electrically conductive layer is formed a photoresist. Above the X-Y stage 31 is disposed a lens 32, and above the lens 32 is disposed a reticle 200 held by a holder 34. Above the reticle 200 is disposed a light source 33 which radiates ultraviolet rays. By radiating ultraviolet rays from the light source 33 to the reticle 200, the circuit pattern formed on the reticle 200 is image-formed on an upper surface of the semiconductor wafer W. Next, the wafer W is exposed to light. Then, the photoresist is developed to thereby leave only portions corresponding to the circuit pattern. By etching the electrically conductive layer using a residual photoresist as a mask, a desired electrically conductive pattern is obtained.
Thus, if the circuit pattern is formed on a glass substrate with low accuracy, for instance, with respect to location of a pattern and dimension of a pattern, final products, namely semiconductor integrated circuit devices, can have only low level of performance or may be non-conforming. Thus, it is quite important to form a circuit pattern on a glass substrate with high accuracy.
A reticle is manufactured generally by means of an electron beam exposure apparatus. FIG. 2 schematically illustrates a conventional electron beam exposure apparatus. On vibration isolators 41 is disposed a vacuum chamber 43 which is reduced in pressure by a vacuum system 42 such as a vacuum pump. In the vacuum chamber 43 is disposed an X-Y stage 44 movable in both X and Y directions, and on the X-Y stage 44 is placed a holder 45 for holding a mask blank 100 from which a reticle will be fabricated. As illustrated in FIG. 3A, the mask blank 100 comprises a glass substrate G on which a chromium layer C is applied, and on the chromium layer C is applied an electron beam resist R.
The mask blank 100 is set on the X-Y stage 44 and fixed with the holder 45 with the chromium layer C and the resist R facing upwardly. Electron beams E radiated from an electron gun 46 is optimized by an electronic optical system 47, and then is radiated on the mask blank 100. Concurrently with the radiation of the electron beams E, data about a circuit pattern to be formed on the mask blank 100 is transformed by means of a data transformer 48 to a format suitable for image-formation. Based on the suitably transformed format, the controller 49 controls the electronic optical system 47 and the X-Y stage 44 to thereby image-form a desired pattern on the mask blank 100. Subsequently to the image-formation, the electron beam resist R is developed, and then the chromium layer C is etched using the residual resist R as a mask and further the resist is removed to thereby form a chromium pattern. Thus, a reticle is completed.
As having aforementioned, when a semiconductor integrated circuit device is manufactured using a thus fabricated reticle, as illustrated in FIG. 1, the reticle 100 is held by the holder 34 in the stepper with the chromium layer C formed on the glass substrate facing downwardly. The reason why the chromium layer C is kept facing downwardly is to avoid debris such as dust from attaching to a circuit pattern formed on the chromium layer C, because if debris is attached to the pattern, images of the debris might be transferred together with the images of the pattern to a semiconductor wafer, and thereby a semiconductor device is made to be non-conforming.
However, since the reticle 200 is held with the holder 34 at peripheral portions thereof, the reticle 200 is bent downwardly at a central portion thereof, namely the reticle 200 has a downward curvature due to a dead weight of the glass substrate. A curvature of the glass substrate generated when the glass substrate is held with the chromium layer C facing upwardly is different from a curvature of the glass substrate generated when the glass substrate is held with the chromium layer C facing downwardly. This is because that a stress of the chromium layer C as well as a dead weight of the glass substrate exerts on the glass substrate.
As aforementioned, the mask blank is held in the electron beam exposure apparatus with the chromium layer C facing upwardly as illustrated in FIG. 2, whereas the reticle 200 is held in the stepper with the chromium layer C facing downwardly as illustrated in FIG. 1. FIG. 3B illustrates a curvature of the glass substrate G held in the electron beam exposure apparatus, whereas FIG. 3D illustrates a curvature of the glass substrate G held in the stepper.
Now, suppose that a circuit pattern P is intended to be image-formed on the mask blank 100 at a distance X1 from a center O of the mask blank 100, as illustrated in FIG. 3A.
In an apparatus for exposing the mask blank 100 to electron beams, since the mask blank 100 is held on the X-Y stage 44, the mask blank is curved so that a central portion thereof lowers or a peripheral portion thereof raises, as illustrated in FIG. 3B. Hence, if electron beams E are radiated to the mask blank 100 at a distance X1 from the center O of the mask blank 100, the circuit pattern P is located at a distance X2 from the center O of the mask blank 100 when the mask blank 100 is released from the holder 45 to thereby return back to its original shape, namely a flat shape having no curvature, as illustrated in FIG. 3C. Suppose that a line connecting the circuit pattern P to the center O of the mask blank 100 makes an angle .theta..sub.1 with a horizontal plane, the distance X2 is represented by the following equation. EQU X2=X1/cos.theta..sub.1 &gt;X1
Thus, the circuit pattern P is dislocated by a distance X from a point where the circuit pattern P is intended to be formed. The dislocation distance X is represented as follows. EQU X=X2-X1=X1(1/cos.theta..sub.1 -1)
On the other hand, in the stepper, the reticle 200 is held at its peripheral portions with the holder 34, and accordingly the reticle 200 is curved so that a central portion thereof downwardly deforms, as illustrated in FIG. 3D. A curvature of the reticle 200 held in the stepper is larger than the curvature of the mask blank 100 held in the electron beam exposure apparatus. Namely, suppose that a line connecting the circuit pattern P to the center O of the reticle 200 makes an angle .theta..sub.1 with a horizontal plane, the angle .theta..sub.2 is larger than the angle .theta..sub.2. Thus, the circuit pattern P is disposed at a distance X3 from the center O of the reticle 200. The distance X3 is represented as follows. EQU X3=X2cos.theta..sub.2
Accordingly, the circuit pattern P is dislocated in the stepper by a distance X4 from a point where the pattern P is intended to be formed. The distance X4 is represented as follows. ##EQU1##
In a reticle composed of quartz glass and having 6 inches of a diameter and 0.25 inches of a thickness, which is so-called "6025 reticle", the above mentioned dislocation is in the range of 0.05 .mu.m to 0.2 .mu.m at a distance of 50 millimeters from a center of the reticle, and in the range of 0.075 .mu.m to 0.3 .mu.m at a distance of 75 millimeters from a center of the reticle. A conventional stepper uses a 1/5scale-down lens, and hence on a semiconductor wafer there is generated a dislocation of one-fifth of the above mentioned dislocation, namely in the range of 0.01 .mu.m to 0.06 .mu.m. A stepper is required to have an alignment accuracy below 0.1 .mu.m for 16 MDRAM level. Accordingly, if there occurs a dislocation ranging up to 0.06 .mu.m due to a curvature of a reticle, it makes a combined curvature ranging up to 0.16 .mu.m, resulting in that it is impossible to conform semiconductor devices to quality standards.
A glass substrate has conventionally had a large thickness for reducing a curvature of a mask blank or a reticle. However, such a large thickness causes the weight of a glass substrate to be heavy and makes the reticle expensive. Furthermore, a stage for the exposure apparatus is required to be large for supporting the heavy weight.
Japanese Unexamined Patent Public Disclosure No. 61-214518 has suggested a method for exposing a mask blank to electron beams with the mask blank having no curvature. However, even if a reticle is manufactured without having a curvature, such a reticle is not useful if a curvature would occur in the reticle when it is used in a stepper.
Japanese Unexamined Patent Public Disclosure No. 3-15065 has suggested a method of compensating for a pattern in dependence on a degree of a curvature which is generated in accordance with an area of a chromium layer. However, this method does not consider a curvature generated due to a dead weight of a glass substrate, and hence if a curvature due to a dead weight of a glass substrate would occur in a stepper, the method would no longer be useful.